In a wireless communications system, in a wireless communications base station system, a baseband processing unit (BBU) performs a baseband algorithm, and may exchange a baseband signal with a remote radio unit (RRU) by using a common public radio interface (CPRI) interface, and then the RRU transmits a downlink transmit signal to an antenna by using a feeder.
However, due to non-ideal factors of some analog components such as a cable, a duplexer, or an antenna feeder, intermodulation such as passive intermodulation (PIM) is generated between multiple downlink transmit signals. In some frequency configurations, an intermodulation frequency may be the same as or close to a frequency of a wanted signal, and lower-order intermodulation generated by a downlink transmit signal overlaps an uplink receive frequency band, that is, a frequency of generated intermodulation interference entirely or partially overlaps an uplink receive frequency. Therefore, interference is caused to the communications system.